In this paper I apply a suitably developed version of Joseph Raz's service conception of authority to the debate over the legitimacy of state action aiming to fulfill cosmopolitan moral obligations. I aim to advance two interrelated theses. First, viewed from the perspective of Raz's service conception of authority, citizens' moral duties to non-compatriots are an appropriate ground for authoritative intervention by agents of the state. Second, international law based on these duties can also enjoy moral authority over government decision makers. An important source of the moral force of international law is derived from the power of international law to improve officials' ability to recognize and conform to the totality of their reasons, which contributes to their states' legitimacy.